Unsteady skin-friction field estimation based on global luminescent oil-film image analysis
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R E G UL A R P A P E R
Taekjin Lee • Chungil Lee • Taku Nonomura
•
Keisuke Asai
Unsteady skin-friction field estimation based on global luminescent oil-film image analysis
Received: 21 April 2020 / Revised: 15 May 2020 / Accepted: 17 May 2020 Ó The Visualization Society of Japan 2020
Abstract A global luminescent oil-film (GLOF) image analysis method to estimate unsteady skin-friction fields in an unsteady flow field is proposed and demonstrated. A governing equation describing the dynamics of the oil film (the thin-oil-film equation) is employed for the unsteady oil-film images. The frequency response of the oil-film movement is analyzed, and a cutoff frequency is defined as a function of the oil-film thickness and the kinematic oil viscosity. The estimating skin-friction vector is defined along with a spatiotemporal weighted window and obtained by solving the overdetermined system of the thin-oil-film equation. The system can be solved by using the weighted linear least-squares method, and the timeresolved skin-friction field can be estimated. The time-resolved GLOF image analysis method is demonstrated on an experiment of a junction flow on a flat surface with a square cylinder. The GLOF images in the Ka´rma´n vortex shedding bounding the flat surface were acquired, and the time-resolved skin-friction fields were obtained. The results showed that fluctuation in the skin-friction vectors corresponds to the shedding frequency, and the vortices bounding the surface were extracted. The averaged skin-friction field is compared with the result of the previous study based on the time-independent model. The normalized skin friction from both methods showed good agreement, which indicates that the quantitative value will be obtained when a calibration process is involved in a future study. Keywords Skin friction Wall shear stress Image-based measurement Oil film Unsteady flow Ka´rma´n vortex shedding
1 Introduction Unsteady flow fields cause various problems in fluid-related machines such as structure fatigues due to the buffet of the wing (Sugioka et al. 2018) and intense aeroacoustic noises generation from Ka´rma´n vortex shedding (Sugioka et al. 2019), and the unsteady flow phenomena should be surveyed for solving the problems. Therefore, the unsteady flow field is one of the interests for the researchers of the measurement and the visualization. In recent years, the pressure-sensitive paint (PSP) measurement has been actively employed for the unsteady flow field (Yorita et al. 2010; Asai and Yorita 2011; Sugioka et al. 2019), while the skin friction measurement has also been applied to unsteady flow fields (Diller and Telionis 1989; Menendez and Ramaprian 1985). Thus far, a flexible film installed an object surface [surface stress-sensitive films (S3F)] has been considered (Crafton et al. 2008) for the measurement method of the unsteady skinT. Lee C. Lee T. Nonomura (&) K. Asai Department of Aerospace Engineering, Tohoku University, Aoba-Ku, Sendai, Miyagi 980-8579, Japan E-mail: nonomura@aero.
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